Alkyl peroxide heat-cured organopolysiloxane elastomers



United States Patent 3,219,726 ALKYL PEROXIDE HEAT-CURED ORGANO-POLYSILOXANE ELASTOMERS Donald L. Bailey, Snyder, N.Y., William T.Black, Stow,

Ohio, and Milton L. Dunham, In, Stamford, Conn., assignors to UnionCarbide Corporation, a corporation of New York No Drawing. Originalapplication Nov. 23, 1954, Ser. No. 470,834. Divided and thisapplication Get. 8, 1964, Ser. No. 402,629

22 Claims. (Cl. 260-825) This application is a division of applicationSerial No. 470,834, filed November 23, 1954.

This invention relates to silicone elastomers and has for an object theprovision of novel products and compositions useful in the production ofsuch elastomers and novel products and compositions resulting from thecuring of heat-curable silicone elastomers. The invention furthercontemplates the provision of novel methods and procedures useful in theproduction of heat-curable compositions and heat-cured compositions.

The invention is based, in part, on our discovery that dialkylperoxides, possess the property or capacity for reacting selectively, orpreferentially, and smoothly and efiiciently to eiiect curing withpredeterminable degrees of crosslinking among and betweenhydrocarbon-substituted linear polysiloxanes contained in heat-curablecompositions largely, or substantially entirely, through the agency ofunsaturated hydrocarbon groups present in the heatcurable compositions.We have found that the capacity of dialkyl peroxides so to function inthe heat-curing of such heat-curable compositions permits the attainmentof results, incapable of attainment heretofore, through the productionand consistent reproduction of heat-cured silicone 'elastomers havingcombined or total or over-all qualities or properties superior to thoseof commercial heatcured silicone elastomers produced heretofore andhaving specific or individual qualities or properties superior to alarge proportion, or substantially all, of the specific or individualqualities or properties of heat-cured silicone elai'torners producedheretofore.

According to most, or virtually all, heretofore customary procedures forproducing silicone elastomers (silicone rubber products) commercially,gum stocks consisting of hydrocarbon-substituted polysiloxanes arecompounded with reinforcing fillers and curing agents on differentialmixing rolls or in mixers, such as the Banbury mixer, of the typesemployed in compounding organic rubber stock. Milling and mixingoperations disperse the fillers and curing agents uniformly throughoutthe gum stock undergoing compounding and the dispersion or distributionof the filler uniformly produces increases in the hardness, tensilestrength, stiffness and resistance to cutting, tearing and abrasion.

Finely divided, solid silicia has been employed most commonly as fillermaterial, alone or in combination with other solid, finely dividedmineral compounds. Commercial producers of silicone elastomers haveendeavored to employ carbon black as a reinforcing filler because of thehighly desirable qualities it imparts to organic rubber products. Thesuccess of such endeavors has been negligible heretofore in spite ofpublished contentions and claims to the contrary.

The polysiloxanes employed in producing silicone elastomers inaccordance with heretofore customary proceice dures, for the most part,are dimethyl polysiloxanes or hydrocarbon polysiloxanes comprisingphenyl pendant groups as well as methyl pendant groups.

It has been proposed, heretofore, to employ hydrocarbon-substitutedpolysiloxanes in which the hydrocarbon substituents consist essentiallyof a large proportion of methyl groups and a relatively small-proportionof vinyl groups. The production 'of such methyl vinyl polysiloxanes wascontrolled to incorporate trifunctional groups through cohydrolysis ofdimethyl-dichlorosilane, methylvinyldichlorosilane vandmonomethyltrichlorosilane in order to pr-oi ide for crosslinking betweensilicon atoms in adjacent polymers through oxygen atoms. Curedelastomers comprising such polymers have not been produced successfullycommercially, possibly, because of interference of the oxygen-siliconcrosslinking with crosslinking through the hydrocarbon groups and,probably, in addition, because of ineliectiveness of 'benzoyl peroxide,the recommended curing agent, in promoting desirable crosslinkinguniformly and eificiently through the pendant hydrocarbon groups.

In the preparation of silicone elastomers is accordance with heretoforecustomary procedures, benzoyl peroxide and tertiary-butyl perbenzoatehave been, and are, employed as curing agents. After compounding of thegum stock, filler and curing agent, the compound or composition issubjected to a mold cure treatment and, thereafter, to a postcuretreatment in an air-circulating oven in order to eliminate volatilematter carried in with the filler and With the gum and to eliminateresidues resulting from the cure reaction. Postcuring usually is carriedout by heating mold cured products at a temperature of 480 F. for aperiod of twenty-four hours.

The following data illustrate the properties of two postcured elastomersprepared in accordance with recipes A and B employed in formingelastomers in accordance with heretofore customary procedures, and onepostcured elastomer prepared by utilizing the recipe (or composition) Cformulated in accordance with the invention and comprising (1) ahydrocarbon-substituted polysiloxane having saturated and unsaturatedhydrocarbon substituent groups and (2) a dialkyl peroxide curing agent.

TABLE I Recipe A:

parts by weight of dimethyl siloxane gum 43 parts by weight of solid,finely divided silica filler 2 parts by Weight of 'benzoyl peroxide.Recipe B:

100 parts by weight of dimethyl siloxane gum (identical with thatemployed in recipe A) 43 parts by weight of solid, finely divided silicafiller (identical with that employed in recipe A) 2 parts by weight oftertiary-butyl perbenzoate. Recipe C:

100 parts by weight of siloxane copolymer contain-' ing 0.15 molepercent of ethyl vinyl siloxane units and 99.85 mole percent(approximately) of dimethyl siloxane units 43 parts by weight of solid,finely divided silica filler (identical with that employed in recipe Aand recipe B) 0.4 part by weight of di-tertiary-butyl-peroxide.

After compounding of the components of the above recipes, the resultingheat-curable compositions were subjected to conventional mold duringtreatments, and the mold-cured compositions were subjected to postcuringtreatments in an air-circulating oven at a temperature of 480 F. forperiods of twenty-four (24) hours with the production of postcuredelastomer compounds (or compositions) having the following properties:

TABLE II Tensile Elongation Hardness Eiastomer compound s'grengt)h(percent) (Shore A) p.s.i.

From Recipe "A 635 210 78 From Recipe B" 531 210 77 From Recipe 895 25078 The above discussion of the conventional curing procedure and thedata indicating results obtainable in practicing the invention, ascompared with results obtainable in carrying out heretofore customaryprocedures, have been presented at this point in the :belief that theirconsideration will simplify the matter of considering the substantialamount of data and the discussions of products and procedures to bepresented hereinafter and, at the same time, will indicate, strikingly,the promise of the invention.

In practicing our invention to produce a silicone gum product that maybe compounded with filler material and a curing agent for ultimateheat-curing, we employ one or more hydrocarbon-substituted siloxaneswhose hydrocarbon substituents comprise saturated hydrocarbon groups ofone or more types and unsaturated hydrocarbon groups of one or moretypes. In the production of heat-curable compounds (or compositions) ofour invention, the hydrocarbon-substituted siloxanes may be employedentirely as linear polysiloxanes, or, they may be employed entirely ascyclic polysiloxanes, or, they may be employed partly as linearpolysiloxanes and partly as cyclic polysiloxanes. When the unsaturatedhydrocarbon substituents are present in a linear polysiloxane theypreferably are present in limited predetermined numbers, and they aredisposed at spaced intervals along the linear polysiloxane chains. Suchlinear polysiloxanes may 'be prepared by copolymerization methods. or byblending methods. Thus, for example, they may be prepared (1) byhydrolysis methods involving cohydrolysis or predetermined proportionsof one or more di-substituted dichlorosilanes whose hydrocarmonsubstituents consist of one or more types of saturated hydrocarbongroups or radicals and one or more di-substituted dichlorosilanes whosehydrocarbon substituents comprise one or more types of unsaturatedhydrocarbon groups or radicals, or (2) by copolymerization ofpredetermined proportions of one or more lowmolecular weight cyclichydrocarbon-substituted polysiloxanes whose hydrocarbon substituentsconsist of one or more types of saturated hydrocarbon groups or radicalsand one or more low-molecular weight cyclic, hydrocarbon-substitutedpolysiloxanes whose hydrocarbon substituents comprise one or more typesof unsaturated hydrocarbon groups or radicals. Blending to achieve theeffect of utilizing a linear hydrocarbon-substituted polysiloxane chainhaving both saturated and unsaturated hydrocarbon substituents may becarried out, for example,

(A) By mechanically mixing (I) one or more cyclic,

hydrocarbon-substituted polysiloxanes whose hydrocarbon substituentsconsist of one or more types of unsaturated hydrocarbon groups orradicals and each of whose molecules contains at least two unsaturatedhydrocarbon groups or radicals and (2) one or morehydrocarbon-substituted linear polysiloxanes whose hydrocarbonsubstituents consist of one or more types of saturated hydrocarbongroups or radicals, or,

(B) By mechanically mixing (1) one or more linear poly 'siloxanes whosehydroearbon substituents consist of one or more types of saturatedhydrocarbon groups or radicals and (2) one or more linear polysiloxaneswhose hydrocarbon substituents comprise one or more types of unsaturatedgroups or radicals and each of whose molecules contains at least twounsaturated hydrocarbon groups or radicals, or,

(C) By mechanically mixing (1) one or more linear polysiloxanes whosehydrocarbon substituents comprise one or more unsaturated hydrocarbongroups or radicals in relatively small proportion and (2) one or morelinear polysiloxanes whose hydrocarbon substituents comprise one or moreunsaturated groups or radicals in relatively large proportion.

The polysiloxanes to be blended may contain any amounts of saturated andunsaturated hydrocarbon substituent groups or radicals that will resultin the production of a blend having the desired proportions of saturatedand unsaturated hydrocarbon groups or radicals. The polysiloxanesemployed in forming blended products may comprise or consist ofrelatively low-molecular weight polysiloxanes or of relativelyhigh-molecular weight polysiloxanes or of both relatively low-molecularweight polysiloxanes and relatively high-molecular weight polysiloxanes.Mixing to eifect blending may be carried out in any suitable manner.Thus, for example, mixing and blending may be carried out on or inrubber stock compounding rolls or mixers either prior to or during themixing and compounding of the polysiloxane stock and the fillermaterial. Mixing to efiect blending may be carried out through the useof solutions or dispersions of one or more of the components to bemixed.

We prefer to employ hydrocarbon-substituted siloxanes whose one or moresaturated hydrocarbon groups consist of types selected from the classconsisting of methyl, ethyl and phenyl and whose one or more unsaturatedhydrocarbon groups consist of types selected from the class consistingof vinyl, allyl and cy-clohexenyl. The saturated hydrocarbon groups maybe present as both of the hydrocarbon substituents of di-substitutedsiloxane units or as single hydrocarbon substituents of di-substitutedsiloxane units or as single hydrocarbon, substituents of di-substitutedsiloxane units the other hydrocarbon substituents of which areunsaturated hydrocarbon groups.

In practicing the invention, we may employ siloxane gums comprising orconsisting of relatively short-chain, low-molecular weight linearpolysiloxanes of chain lengths falling within a limited range, whichgums are pourable liquids, or, we may employ diiferent siloxane gums ofdifferent higher viscosities each of which diiferent gums comprises orconsists of relatively longer-chain, relatively higher-molecular weightlinear polysiloxanes, also hav ing chain lengths falling within alimited range, up to the point at which the viscosity of the gum is suchthat it approaches the solid state and will barely flow when unconfined.

As curing agents, we prefer to employ dialkyl peroxide compounds havingcompositions corresponding to the following structural formulas:

wherein R represents the same alkyl group throughout or alkyl groups oftwo or more different types and n is zero (0) or a larger integer.

Among the specific curing agents that we prefer to employ are included:

Di-terti-ary-butyl peroxide;

T ertiary-butyl-t-riethylmethyl peroxide; and

Tertiary-butyl-tertia-ry-triptyl peroxide, the composition of which isrepresented by the structural formula In producing silicone elastomers(silicone rubber) in accordance with our invention, we may employ any ofthe filler materials consisting of inorganic compounds or any suitablecombination of such filler materials employed in the production ofelastomers in accordance with heretofore customary procedures.

We may employ, also, carbon black fillers.

Following are two lists of suitable fillers consisting of inorganiccompounds, one setting forth filler names and properties and the othersetting forth the filler names, manufacturers and chemical compositions.

FILLER PROPERTIES Particle Surface Filler diameter area, sq. m. Approri-Sp. Gr.

(milliper gram mate pH microns) Santocel CS 30 110-150 3. 5-4. 5 2. 2Hi-Sil X303 20-25 140-160 7.0-8. 1. 95 Degussa Aerosil 15-20 175-200 4.-6. 0 2. 0 Du Pont very fine silica 275-300 7. 5-9. 5 1. 98 Celite 2701, 000-6, 000 a. 7. 0 2.15 Celite superfloss.-. 2, 000-4, 000 '20 8. 52. 3 Witcarb R 30-50 32 11.3 2. 65 FeO RY-2196 1, 000 4. 95 Titanox RA300-400 7. 0 4. 2 Alon 20-40 50-120 4. 5-7. 0 3. 6 TAM Superpa 5, 000 4.5

DESCRIPTION OF VARIOUS MATERIALS USED FoR ELASTOMER FILLERS Trade nameChemical composition Santocel CS Finely divided silica. Aerosil orCabosiL. Finely divided very pure silica.

Hi-Sil X-303 Finely divided, SlllO-l. Dicalite PS Diatomaceous silica.Superfloss Do.

Celite 270- Do.

Iceberg- Aluminum silicate.

Witcarb R Calcium carbonate.

Zinc Oxide.-. Zinc oxide.

Superpax- Zirconium silicate.

'litanox RA Titania.

Lithopone Barium sulfate-zinc sulfide.

Du Pont very Iron oxide. Aluminum oxide Finely divided silica.

We may employ any suitable carbon black as filler in preparing siliconeelastomers in accordance with the invention. Furnace blacks can beemployed satisfactorily, particularly high-abrasion furnace blacks, inelastomer production procedures involving compounding silicone gum withcarbon black and a dialkyl peroxide curing agent, subjecting thecompound or composition to a mold cure and, thereafter, subjecting thecured product to postcure heat-aging treatment.

We have found that the capacity of an avaiiab'le carbon black product tofunction effectively as a filler is iniiuenced by particle size,hydrogen ion concentration and content of volatile matter. Carbon blackproducts consisting of particles larger than about 850 A. may providelow reinforcement. Generally, carbon black products consisting ofparticles larger than 850 A. or smaller than 300 A. and having volatilematter contents higher than about 2.0 percent by weight and hydrogen ionconcentrations lower than that corresponding to a pH of about 9.0 (asindicated by results obtained in measuring hydrogen ion concentrationsof water dispersions of carbon blacks in accordance with the standardprocedure employed by the carbon black manufacturers) can not beemployed advantageously without having been subjected to preliminary orprecure corrective treatments,

such, for example, as precure heat-aging treatments, or in compoundingtreatments in recipes that include an alkaline agent or acid acceptor.Channel blacks, being acid in reaction and having relatively highcontents of volatile matter require suitable preliminary or precuretreatments to condition them for effective use.

In the use of carbon black fillers in practicing our invention, weprefer to employ furnace blacks comprising particles in the range, 300A. to 850 A., containing volatile matter in amounts not greater thanabout 2.0 percent by weight and having hydrogen ion concentrationscorresponding to a pH not lower than about 9.0. We may employ channelblack products in procedures involving precure corrective treatments,such for example, as precure heat treatments, and in proceduresinvolving incorporation in recipes for compounding of alkaline agents oracid acceptors such as calcium carbonate.

Preparation of hydrocarbon-substituted linear polysiloxanes of theinvention containing saturated and unsaturated hydrocarbon substituentsmay be carried out by means of any of the procedures whose fundamentalfeatures are known to those skilled in the art. In practicing ourinvention for the production of such linear polysiloxanes, in one of itsaspects, we follow a procedure involving (1) hydrolysis, on the onehand, of one or more hydrocarbon-substituted dichlorosilanes in whichthe substituents consist of saturated hydrocarbon groups to produce acrude hydrolyzate containing linear polysiloxanes whose hydrocarbonsubstituents consist of saturated hydrocarbon groups of one or moretypes, and, on the other hand, of one or more hydrocarbon-substituteddichlorosilanes whose hydrocarbon substituents comprise one or moreunsaturated hydrocarbon groups to produce a crude hydrolyzate containinglinear polysiloxanes whose hydrocarbon substituents comprise unsaturatedhydrocarbon groups of one or more types, (2) depolymerization of thecrude hydrolyzates to form separate mixtures of lowboiling point,low-molecular weight, cyclic polymers (in one case having only saturatedhydrocarbon pendant groups and in the other case having some unsaturatedhydrocarbon pendant groups) and undesirable material resulting from thevirtually unavoidable presence of monofunctional and trifunctionalsilanes in association with the difunctional dichlorosi'lane productstreated initially and (3) fractional distillation of the two products ofdepolymerization to vaporize and collect two pure products containingthe low-boiling point, low-molecular weight cyclic polymers free of anysignificant amounts of monofunctiona-l and trifunctional groups ofmolecules. We utilize the two pure, or relatively pure, productsobtained in the distillation treatments, one comprising cyclic polymerswhose pendant groups consist essentially of one or more saturatedhydrocarbons and the other comprising cyclic polymers whose pendantgroups comprise one or more unsaturated hydrocarbons, to produce alinear polysiloxane copolymer by mixing them in proportions such as toprovide in the resulting linear polysiloxane product a predeterminednumber of unsaturated groups with respect to the total number ofhydrocarbon groups present therein, and we subject the mixture to anappropriate polymerization treatment under controlled conditions toproduce linear polysiloxanes of desired molecular weights and viscositieand having both saturated and unsaturated pendant groups attached tosilicon atoms along the linear polysiloxane chains.

Thus, for example, in the production of a copolymer comprising a linearpolysiloxane having both saturated and unsaturated hydrocarbon groupsattached to silicon atoms along the linear chain, a copolymer havingmethyl and ethyl (saturated groups) and vinyl (unsaturated groups) maybe produced by subjecting to hydrolysis a product comprisingdimethyldichlorosilane (which product will, normally, be contaminatedwith about 0.3 percent to 0.7 percent by Weight ofmonomethylt-richlorosilane) to produce a crude hydrolyzate containingpolymerized dimethyl linear polysiloxanes and undesirable materialsresulting from the presence in the dimethyldichlorosilane product ofmonomethyltrichlorosilane. The crude hydrolyzate, subsequently, issubjected to a depolymer-ization treatment by mixing it with potassiumhydroxide (KOH) and diphenyl solvent in the proportions, by weight, 39parts of the crude hydrolyzate, 1.0 part of potassium hydroxide and 60parts of diphenyl solvent, and heating the mixture at a temperature inthe range, 150 C. to 175 C., under an absolute pressure of 100 mm. Hg toproduce and remove by vaporization a product consisting of low-molecularweight cyclic polysiloxanes, comprising, for example, about eighty-fivepercent (85%) of the tetramer and fifteen percent (15%) of mixed timer[(CH SiO] and pentamer [(CH SiO] Materials resulting from the presenceof monomethyltrichlorosilane (MeSiCl in the initial product containingdimethyldichlorosilane remain as a residue in the distillation vessel ortower.

The distillate consisting essentially of low-molecular weight cyclicdimethyl polymers, free of any significant amount of monofunctional andtrifunctional groups or molecules, is prepared for a controlledpolymerization treatment in a polymerization mixture formed by mixingthe distillate with (1) one or more cyclic ethyl vinyl polysiloxanessuch, for example, as the trimer or both, (2) potassium silanolate,

CH3 K(S i-O)nOK in amount suflicient to provide thirty (30) parts byweight of potassium ion (K+) per million parts by weight of thedistillate, and (3) an amount of one or more monofunctional compoundscalculated to function as end-blockers for limiting the degree ofpolymerization and, consequently, the lengths and molecular weights ofthe linear polysiloxane chains and for stabilizing the polymers. Afterthorough stirring in order to effect thorough dispersion of thecomponents and the production of a substantially homogeneous product,the mixture is heated in a sealed vessel at a temperature of about 150C. for a period of time varying from about one hour to two and one-halfhours. The degree of completion of the polymerization reactions isdetermined through viscosity measurements or miniature penetrometerreadings. The elimination of residual trifunction compounds, also, ispromoted by the use of monofunctional compounds in accordance with ourinvention.

'M'onofunctional compounds that may be employed satisfactorily forcontrolling polymer growth include, among others,

Hexa-methyl disiloxane, (CH SiOSi (CH Tetramethyldiethoxy disiloxane,

3)z( 2 5 2 5) 92 Monoethyltriethoxy silane, C H Si(OC HDiethyletetraethoxy disiloxane,

C 'H (C H O SiOSi (OC H C H Tetramethyldimethoxyethoxy disiloxane,

(CH CH OC II OSiOSiOC H OOH (OH and Divinyltetraethoxy disiloxa-ne,

- CH =CH(C H O) SiOS i(OC H 0H=CH Copolymers whose pendant groupsconsist largely of methyl groups and include additional saturatedhydrocarbon groups other than, or in addition to, ethyl groups andunsaturated hydrocarbon groups other than, or in addition to, vinylgroups may be employed by means of pro cedures similar to that describedabove or by means of procedures modified in accordance with the knowncharacteristics of the various hydrocarbon groups sought to be included.

Processes employed in producing copolymer gums of our invention for usein producing compounds and elasto mers of our invention preferably areso controlled as to produce gums having R (saturated) R (unsaturated)siloxane units disposed along the linear polysiloxane chains in amountsin the range, 0.037 to 0.74 mole percent (equivalent to about 0.05 to1.0 weight percent), of the total number of moles contained in thelinear polysiloxane chains. Stated otherwise, from about 0.037 to about0.74 percent of the total silicon atoms in the copolymer gums of thisinvention are bonded to unsaturated hydrocarbon groups. The introductioninto the polysiloxane chains of the numbers of unsaturated hydrocarbongroups indicated contemplates the provision of sufiicient numbers ofsuch groups to provide for the development, upon curing, of about five(5) to twenty (20) crosslinks per molecule through the unsaturatedgroups. It is to be understood that we may provide unsaturatedhydrocarbon groups in greater or lesser numbers to provide forestablishing crosslinks through such groups in greater or lesser numbersin pacticing our invention.

Blending processes and operations carried out in accordance with ourinvention are based, generally, on the same considerations with respectto unsaturated hydrocarbon groups as are processes and operationsinvolving the production and use of copolymers containing unsaturatedhydrocarbon groups.

In producing hydrocarbon substituted polysiloxane gums in which thehydrocarbon substituents consist of saturated hydrocarbon groups, foruse in the blending processes of our invention, we may employ theprocedure described above for producing copolymers modified only by theexclusion of cyclic polysiloxanes containing unsaturated hydrocarbongroups from the mixture.

The hydrocarbon-substituted linear polysiloxane polymers and copolymerswe employ, or produce, or produce and employ in carrying out ourinvention preferably are produced under conditions so controlled as toavoid (l) the incorporation therein of any significant amounts oftrifunctional compounds, groups or molecules, thus to avoid crosslinkingof polysiloxane linear chains through silicon and oxygen atoms insubsequent curing operations, and (2) the incorporation therein of anysignificant amounts of monofunctional compounds, groups or moleculesother than those specifically provided to serve as end-blockers forlimiting the degree of polymerization and for stabilization purposes.Accordingly, such linear polysiloxane polymers and copolymers containhydrocarbon pendant groups and silicon atoms in the ratio of 2.0hydrocarbon groups per silicon atom (approximately). Deviation from aratio of 2.0 in any instance with respect to the preferred practices,procedures, gums and compounds of our invention will be insignificantfor all practical purposes since it will be attributable to the presenceof end-blocking hydrocarbon groups whose total numbers will beinsignificant as compared with the total numbers of hydrocarbon groupsattached to silicon atoms of the linear polysiloxane chains between thesingle hydrocarbon groups serving as blockers for each end of each ofthe polysiloxane chains.

We prefer to employ polysiloxane polymers and copolymers containinghydrocarbon pendant groups in a ratio of two (2.0) per silicon atom inorder to provide for optimum effectiveness of crosslinking inheat-curing through the agency of unsaturated hydrocarbon groups alone.It is to be understood, however, that, because of the effectiveness ofthe dialkyl peroxide curing agents we employ, results superior to thoseattainable theretofore can be atatined in practicing our invention evenwhen the ratio of hydrocarbon groups to silicon atoms is slightly lessthan 2.0 because of the presence of minor amounts of tn'functionalgroups which permit a limited amount of crosslinking through silicon andoxygen atoms in heat-curing treatments.

For curing purposes, we may employ the curing agents of the invention inany suitable amounts and proportions with respect to the amounts andproportions of unsaturated groups present in copolymers and in blendedproducts, but we prefer to employ the curing agents in stoichiometric orchemically equivalent amounts with respect to the unsaturatedhydrocarbon groups plus, when necessary, amounts in excess of thestoichiometric amounts suthcient to compensate for volatilization of thecuring agents that might take place during heatcuring operations.

In producing elastomers in accordance with our invention, we prefer tosubject compounds (or compositions) that have been milled or otherwisemixed to disperse fillers and curing agents in the polysiloxanecopoly-mer gums and blended gums to mold-curing (or heat-curing)treatments at temperatures above 300 F. (preferably 340 F. or higher)for periods of time longer than fifteen minutes (preferably twenty-fiveminutes or longer), and, thereafter, to subject the mold-cured (orheat-cured) products to heat-aging treatments at higher temperatures inair-circulating ovens for periods of time sufiiciently long to permiteffective elimination through vaporization of objectionable inclusionssuch as water, residues from curing agent reactions and low-molecularweight gum stock fractions. Heating of the cured prod nets to atemperature of about 480 F. through or during a period of abouttwenty-four hours usually is effective for eliminating such undesirableinclusions.

In some instances, we find it to be advisable, in order to provide forobtaining optimum results, to subject a compound, after incorporation ofthe filler in the gum but before inclusion of the curing agent, to roomtemperature aging for a period of about one day to one week, or, toprecure heat-aging at an elevated temperature above about 250 F. and upto 300 F. or higher for a period of time ranging from one to two or morehours. Both types of precure aging provide opportunities for betterwetting of fillers by the gums, and the precure heat-aging provides theadditional advantage of effecting elimination of objectionable volatilematters such as water and adsorbed gases carried into compounds bytillers.

At the conclusion of a precure aging treatment, the compound isre-plasticized, as by milling, the curing agent is incorporated toproduce a heat-curable compound and, thereafter, the heat-curablecompound is heat-cured, and the heat-cured product is subjected to apost-cure heat-aging treatment. As hereinbefore pointed out, precureheat-aging of a compound comprising a carbon black filler, not amenableto use as a filler for a compound to be subjected to immediateheat'curing, permits its use elfectively when the compound isre-plasticized, inoculated with a curing agent and, thereafter,subjected, successively to heat-curing and heat-aging treatments.

Precure heat-aging treatments are not essential, but may be employed,when a compound comprising any suitable filler is to be formed inthick-section moldings. Precure heat-aging permits the elimination ofvolatile matters at a stage wherein distortion resulting from gaselimination is not harmful and reduces the amount of gas that must beeliminated in the postcure heat-aging with consequent reduction in theamount that must be eliminated at the critical time when density andstructural form must be retained.

The following examples describe processes of the invention involving theproduction and use of linear polysiloxane gums of different compositionsand consistencies in accordance with the invention:

Example I A relatively hard gum comprising linear polysiloxanesconsisting of dimethyl siloxane units (99.926 mole percent) and ethylvinyl siloxane units (0.074 mole percent) was prepared by heating 3065grams of octamethylcyclotetrasiloxane tetramer [(CH SiO] to atemperature of C. and, thereafter, adding ethyl vinyl trimer in amountequal to one-tenth of one percent (0.10%) of the weight of the resultingmixture and an amount of potassium silanolate, K(OSiMe OK, containingfourtenths of one percent (0.4%) by weight of potassium ion (Ksufiicient to provide 27.8 parts of potassium ion per million parts ofthe mixture of cyclic siloxanes. The resulting mixture was stirred forten minutes to effect thorough intermixing of the components, and,thereafter, it was heated at a temperature of C. for two and one-halfhours in a sealed vessel. After the conclusion of the heating period,the vessel was cooled to room temperature and opened. The cooled vesselcontained a linear polysiloxane gum having a hardness corresponding to aminiature penetrometer reading of 35 at room temperature, which wassoluble in toluene and whose pendant hydrocarbon group to silicon atomratio was 2.0 (approximately) An elastomer was prepared by Banburymixing 900 g. of the linear polysiloxane with 315 g. of Santocel CS(finely divided silica) filler and then milling and sheeting off a 6"x12 roll mill. The resulting compound was then precure heat-aged for onehour at 150 C. in a circulating air oven. One hundred thirty-five gramsof the precure heat-aged compound was replasticized with 0.4 gram ofdi-tertiary-butyl peroxide. The resulting compound was heat-cured in amold at 340 F. for 25 minutes and postcure heat-aged at a temperature of480 F. (250 C.) for 30 hours.

The mold-cured and postcure heat-aged products had the followingphysical properties:

Mold-cured for 25 Postcure heat-aged minutes at 350 F.

for 30 hours at 480 F Tensile, p.s.i 1, 136 785 Elongation, percent 720575 Set at break, percent- 3 3 Hardness, Shore A Durometer 26 30 Example2 Mold-cured for 30 Postcure heat-aged minutes at 350 F. for 24 hours at480 F.

Tensile (p.s.i.) 825 040 Elongation (percent) 950 625 Setat break(percent) 15 12 Hardness (Shore A) Durometer 38 55 Example 3 Thefollowing data relating to physical properties were obtained in curingcompounds of the compositions and under the conditions indicated,employing tertiary-butyl peracetate as curing agent.

RECIPES (IN PARTS BY WEIGHT) Gum stock consisting essentially ofdimethyl polysiloxane units (99.65 mole percent) and ethyl vinylsiloxane units (0.35 mole percent) Gum stock consisting essentially ofdimethyl Properties of product of immediate cure:

Tensile (p.s.i.) Elongation (percent) Hardness (Shore A) Set at break(percent) Properties of products posteure heataged at 480 F. for 24hours:

Tensile (p.s.i.) Elongation (pereent) Hardness (Shore A)- Set at break(percent) Linear shrinkage (percent Weight loss (percent) Example 4 Arelatively soft ethoxy end-blocked gum comprising linear polysiloxanesconsisting of dimethyl siloxane units (99.74 mole percent) and ethylvinyl siloxane units (0.26 mole percent) was prepared by mixing 29,800grams of octamethylcyclotetrasiloxane [(CH SiO] with 4 grams oftetramethyldiethoxydisiloxane and 300 grams of a polysiloxane copolymerconsisting of ethyl vinyl siloxane units (28 percent by weight) anddimethyl siloxane units (72 percent by weight), stirring to mix thecomponents thoroughly, heating the mixture to a temperature of 145 0,adding as a catalyst a solution of potassium silanolate in amountsufficient to provide 30 parts of potassium ion per million parts of themixture, again stirring for ten minutes, and, then, heating theresulting mixture in a sealed vessel to a temperature of 150 C. for anhour and fortyfive minutes. After the conclusion of the heating pe-'riod, the vessel was permitted to stand overnight to permit cooling ofits contents to room temperature.

The linear polysiloxane gum contained in the cooled vessel had ahardness corresponding to a miniature penetrometer reading of 78 at roomtemperature, was soluble in toluene and had a pendant hydrocarbon groupto silicon atom ratio of 2.0 (approximately).

The gum was compounded in accordance with the following recipes (allparts refer to parts by weight):

Recipe and Parts of Parts of Parts of Parts of compound Parts SantocelSuperbenzoyl di-tertiarynumber of gum CS floss peroxide butyl peroxideCOMPOUNDING PROCEDURE FOR THE ABOVE RECIPES (1) Recipes specifying onlySantocel CS filler.-Three batches each containing 300 grams of gum werecompounded for each recipe, five different recipes on the basis offillers. In compounding each batch, the gum (300 grams) was sheeted on a6 inch x 12 inch two-roll mill with rolls at room temperature; Filleraddition was begun immediately after the sheeting of the gum on the millwas completed. The filler was added in portions of about 20 grams each(more or less).

The time consumed in adding and completely dispersing the filler was notallowed to exceed fifteen minutes. At the end of the fifteen minuteperiod, the compound was sheeted from the mill. After all three batchescontaining the gum and filler for each recipe had been compounded, allwere placed on the mill again and homogenized for fifteen minutes toform a single batch for each recipe. Each homogenized batch was sheetedfrom the mill, rolled into a tight roll, and all were stored for 48hours at room temperature. At the conclusion of the storage period, eachbatch was split into two parts. One part was placed on a cold 6 inch x12 inch two roll mill and re-plasticized. This process required about 20minutes. The compound was sheeted from the mill and weighed. Thequantity of di-tertiary-butyl peroxide required for curing was estimatedto be 0.8 part on the basis of parts gum content. The samere-plasticization procedure was followed with the other part of eachbatch. The benzoyl peroxide requirement for curing was estimated to be0.4 part per 100 parts of gum. Di-tertiary-butyl peroxide was dispersedin half of the batches and benzoyl peroxide was dispersed in the otherhalf of the batches by milling. The compounds having the curing agentsdispersed therein were sheeted from the mill which was set to producesheets about one-tenth of an inch (0.1") thick. Slabs weighing 63 gramsand about five inches square were cut from the sheets and cured in a 6inch x 6 inch mold. Compounds containing benzoyl peroxide as curingagent were cured for 15 minutes at 250 F. Slabs containingdi-tertiary-butyl peroxide were cured for 20 minutes at 340 F.

(2) Recipes specifying Santocel CS and superfloss. The proceduredescribed above for Recipes specifying only Santocel CS filler werefollowed exactly except that the time allowed for filler addition was 20minutes instead of 15 minutes.

Two compression set plugs /2 inch thick and 1.129 inches in diameter)were cured from each of the compounds cured with di-tertiary-butylperoxide.

Similar, but laminated, compression set plugs were prepared fromelastomers cured with benzoyl peroxide.

The cure cycle for the compounds cured with diter-tiarybutyl peroxidewas 1 hour at 340 F. They were postcure heat-aged at 480 F. for 24hours.

The cure cycle for the compounds cured with benzoyl peroxide was 15minutes at 250 F. They were postcure heat-aged at 480 F. for 24 hours.

Portions of two compounds having di-tertiary-butyl peroxide incorporatedtherein and two compounds having benzoyl peroxide incorporated thereinwere subjected to thick-section curing in a 1 inch x 3 inch diametermold.

The cure cycle for the thick section-cure with di-tertiarybutyl peroxidewas 2 hours at 340 F. Postcure heataging was carried out for 24 hours at480 F.

The cure cycle for the thick-section cure with benzoyl peroxide was 2hours at 250 F. Postcure heat-aging was carried out for 24 hours at 480F.

The data obtained in testing the various elastomers produced in curingthe compounds 'are listed immediately below. In order to facilitatecomparison of results obtamed in curing the five different pairs ofcompounds where the two members of each pair are identical except thatin one di-tertiary-butyl peroxide was employed as curing agent and inthe other benzoyl peroxide was employed as curing agent, the pairs areset forth as such with the properties of elastomers obtained by curingwith di-tertiary-butyl peroxide being set forth above the properties ofelastomers obtained by curing with benzoyl peroxide. The compositions ofthe compounds employed are listed near the commencement of this exampleunder the identification symbols set forth below.

ELASTOMER PHYSICAL PROPERTIES A/5F 5B/5G 5C/5H 5D/5J 5E/5K Curing agentMold Cure:

Tensile (psi) 916/922 1, 000/890 1,000/1, 059 922/930 936/1, 000 DTBP/BZO: Elongation (percent) 355/370 335/345 310/345 330/377 270/370DTBP/BZrOZ Hardness (Shore A) /39 /44 47/46 /52 72/70 DTBP/BZ1O2 P tSetat Break (percent) Nil/Nil Nil/Nil Nil/Nil Nil/Nil Nil/Nil DTBP/BZzOg 0scure:

Tensile (p.s.i.) 723/889 856/754 720/884 946/945 799/781 DTBP/BZQOZElongation (pcrcent) 250/290 230/240 230/230 220/250 160/200 DTBP/BzgozHardness (Shore A) 53/50 57/55 /60 71/68 85/81 DTBP/BZQOQ Set at Breakil/Nil Nil/Nil Nil/Nil DTBP/BZQOZ Weight loss (percent) 7. 1/7. 9 b.9/7. 8 6. 6/7. 2 DTBP/Bzzoz Total shrinkage (percent) 7. 3/0. 2 6. 2/6.2 8.3/7. 3 DTB P/BZ202 Tear (Die 0) (pounds per inch) 39/38 35/35 53/53D'IBP/BzzOr 100% modulus (p.s.i.) 185/172 230/213 484/394 DTBP/BzgOzCompression Set (22 hours at 350F. Method B) (percent), 12. 7/18. 3 13.2/18. 7 2 19. 9/28. 1 DTBP/BzzOz Compression Set (96 hours at 350F.Method 13) (percent). 25. 5/31. 4 28. 8/32. 5 31. 5/35. 7 DTBP/BZEOZThick-section cures:

(Unaged) (hardness, surface) 39-42/44-6 2/6971 DTBP/BzzOz (Hardness,Center) 40-2/44-6 71%2/6941 DTBP/BZ'JO2 (Aged) 24 hours at 480F 48/45-6/73 DTBP/BZzOz (Hardness center) 48/30 05-7/68 DTBP/BZaOz Remarks-VisunlObservations:

5B, 5ENo discoloration, delamination or cracks. 5GMuch sponging,cracking. 5K-Delarnination, no discoloration.

No thicksection cures were carried out for 5A, 50, 5D, SF, 511 and 57.

1 DTBP=di-tertiary-butyl peroxide; B2202=b8ilZOYl peroxide.

2 High compression sets probably can be attributed to filler loadings.

Example 5 For the purpose of demonstrating the capacity of linearpolysiloxane gums of the invention to function to produce elastomershaving very low compression set, parts of a linear polysiloxanecopolymer consisting essentially of dimethyl siloxane units (99.75% byweight) and ethyl vinyl siloxane units (0.25% by weight:0.185 molepercent) was milled with 45 parts of Santocel CS and 1.0 partditertiary-butyl peroxide (all parts by Weight) and the resultingcompound was cured by heating to 340 F. for thirty (30) minutes, and thecured product was subjected to a postcure heat-aging treatment for 24hours at 480 F. (250 C.).

The compression set of the postcure heat-aged elastomer was shown to be9.7 in tests conducted in accordance with ASTM D39552T method B for 22hours at 350 F.

Example 6 For the purpose of demonstrating the capacity of linearpolysiloxanes of the invention to function to pro duce elastomcrs thatcan be employed successfully to make sound thick-section moldings, apolysiloxane copolymer gum consisting essentially of dimethyl siloxaneunits (99.8 percent by weight) and ethyl vinyl siloxane units (0.20percent by weight:0.148 mole percent) was milled with Santocel CS todisperse the SantOcel CS uniformly in the polysiloxane gum. Theresulting compound was precure heat-aged for 2 hours at 300 F. and,thereafter, remilled with di-tertiary-butyl peroxide. The componentswere employed in the proportions by weight, 100 parts of gum, 43 partsof Santocei CS and 0.7 part of di-tertiary-butyl peroxide. The resultingcompound was heat-cured at a temperature of 340 F. for thirty minutes.The cured gum showed tensile of 890 p.s.i., elongation of 220%, andShore A D'urometer hardness of 6 5. Compression set, ASTM D39552T,method B, was 15%. (22 hours at 350 F).

Uncured compound of the above description was molded into a 3 inchdiameter x 1 inch high cylinder and cured at 340 F. for one and one-half(1 /2) hours and subjected to a postcure at 480 F. for 24 hours. Themolding was solid, with no blowing or other defects, and its Shore ADurorneter hardness values ranged from 47 to 50 over a cross section.

After heat-aging for 18 hours at 480 F. (250 C.), the disc was cut inhalf vertically with respect to its plane faces. There was a slightdiscoloration in the center, but hardness still was 47 to 50, and therewas no evidence of blowign or degradation.

Example 7 For the purpose of demonstrating the capacity of linearpolysiloxane copolymers of the invention to function to provideelastomers having carbon black fillers, the polysiloxane copolymer gumemployed in Example 1 was milled with carbon black (Philblack A, MAPgrade) and di-tertiary-butyl peroxide curing agent to effect uniformdispersion of the carbon black and the curing agent in the gum. Thecomponents were employed in the proportions by weight, 100 parts of gum,60 parts of carbon black and 2.8 parts of curing agent. The milledproduct yielded a definitely cured elastomer when heat-cured in a moldat 350 F. for thirty (30) minutes.

Example 8 The linear polysiloxane copolymer gum employed in Example 4was milled with carbon black (Philblack A, MAP grade) anddi-tertiary-butyl peroxide curing agent to effect uniform dispersion ofthe carbon black and the curing agent in the gum. The components wereemployed in the proportions by weight, 100 parts of gum, 47 parts ofcarbon black and 0.8 part of curing agent. The product of the millingtreatment, when heat-cured in a mold at a temperature of 340 F. fortwenty-five (25) minutes, yielded a mold cured, carbon black-filledelastomer having the following physical properties:

Hardness (Shore A) 4S Tensile strength (p.s.i.) 737 Elongation (percent)300 Example 9 In another demonstration of the capacity of polysiloxanecopolymers of the invention to function to provide carbon black-filledelastomers, a linear polysiloxane copolymer gum consisting essential ofdimethyl siloxane units (99.5 percent by weight) and ethyl vinylsiloxane units (0.5 percent by weight:0.37 mole percent) was milled withcarbon black (Philblack A, MAP grade) and di-tertiary-butyl peroxidecuring agent to eifcct uniform dispersion of the carbon black and curingagent in the gum. The components were employed in the proportions byweight, 100 parts of gum, 60 parts of carbon black and 2.8 parts ofcuring agent. The product of the milling treatment, when heat-cured in amold for 30 minutes at 350 F. and subsequently subjected to a postcureheat-aging treatment for 24 15 hours at 480 F. (250 C.), yieldedcarbon-filled clastomers having the following physical properties:

As shown by the data set forth herein, polysiloxane products containingethyl vinyl siloxane units may be heat-cured advantageously with organicperoxides generally to produce superior silicone elastomers, and ourinvention is based, in part, on that discovery. The organic peroxidesthat we have found to be effective in curing polysiloxanes containingethyl vinyl siloxane units, in addition to dialkyl peroxides, arealiphatic peroxy esters, represented by tertiary-butyl peracetate;aromatic peroxy esters, represented by tertiary-butyl perbenzoate; anddiacyl peroxides, represented by di-benzoyl peroxide and hisdichlorobenzoyl peroxide. As in the case of ditertiary-butyl peroxide,tertiary-bu-tyl peracetate is highly effective in curing polysiloxanescontaining dimethyl siloxane units and ethyl vinyl siloxane units andrelatively ineffective in curing polysiloxanes consisting of dimethylsiloxane units, whereas, tertiary-butyl perbenzoate effectively curesboth polysiloxanes containing dimethyl siloxane units and ethyl vinylsiloxane units and polysiloxanes consisting: of dimethyl siloxane units.The relatixe capacities of curing agents to effect curing ofpolysiloxanes consisting of dimethyl siloxane units and polysiloxanescomprising pendant vinyl groups are illustrated roughly by the diagramshown below.

peratures are between 150 C. and the temperature at which postcureheat-aging is carried out, usually, 250 C. (480 F.). Of course, it is tobe understood that we do not advocate the use of a peroxide curing agentthat will react explosively at the temperature employed in the mold-cureoperation, or in a postcure heat-aging operation when an excess beyondthat which will react completely is incorporated in the compound to besubjected to a mold-cure operation.

A polysiloxane containing an unsaturated hydrocarbon group of any sizemay be employed in practicing the invention if the unsaturated groupcontains a double bond in a position in which it is accessible forreaction. Among the unsaturated hydrocarbon substituent pendant groupsthat we prefer to employ are those that result in the production ofhydrocarbon linkages among the chains of linear polysiloxanes.

As hereinbefore stated, use of the procedures, processes, polysiloxanegums and various compositions and compounds of the invention,facilitates the attainment of many advantageous results with respect toprocedural matters, with respect to product quality and with respect tocompositions and physical properties of silicone elastomers incapable ofattainment through the use of the heretofore customary procedures,processes, polysiloxane gums, compositions and compounds.

Among the deficiencies of processes and procedures employed heretoforeare (1) inability to produce elastomers having uniformly low compressionsets, (2) inability to employ carbon black fillers effectively with theproduction of carbon black-filled elastomers, (3) inefficient curingagent utilization, (4) inability to control crosslinking effectively and(5) inability consistently to mold-cure and postcure heat-agedthick-section elastomers.

The compression sets of silicone elastomers produced IHDROCARB ON GROUPSUSCEPTIBILI'EY Me thy].

.. fi i i fi fl A'IiphatIo Groups oer Good can Cure Home 1 Terti141111211 Ter CURING AGENT butyl Peroxide auicrrvrrx The above diagramshows the methyl group to be least susceptible to curing of thesaturated aliphatic groups and the vinyl group to be more susceptible tocuring than the saturated aliphatic groups. Benzoyl peroxide is shown asthe most highly reactive and di-tertiary-butyl peroxide is shown as theleast reactive of those curing agents set forth in the diagram.

In practicing our invention, we prefer to employ peroxide curing agentsthat react effectively at mold-cure temperatures but whose vaporpressures at those temperatures are substantially lower than 760 mm. Hg.We can employ advantageously, however, such curing agents whose boilingtemperatures at 760 mm. Hg are between 100 C. and 150 C, and such curingagents whose boiling term in accordance with heretofore customaryprocedures are undesirably high, being usually above seventy percent,except when undesirable additive agents such, for example, as toxicchernical compounds of mercury and cadmium are employed. Such compoundscan not be eliminated in initial curing operations or in postcureheating operations, and their presence in silicone elastomers may behighly objectionable, particularly when the elastomers are to beemployed for food and pharmaceutical uses.

The curing agents employed heretofore have been confined largely toclasses of organic peroxides such, for example, as benzoyl peroxide andtertiary-butyl perbenzoate, which are strongly oxidizing. The presenceof carbon black in a compound to be cured interferes with the action ofthe curing agent and results in the production of elastomers having poorphysical properties.

Since the eificiency of utilization of the curing agent is poor,relatively large amounts must be employed. The use of excess curingagent results in the production of excessive amounts of harmfulresidues, including benzoic acid which functions as a silicone gum stockdepolymerization catalyst.

Also, because of the efiiciency of utilization of curing agent, thecrosslinking reaction is difficult or impossible to control accurately,and crosslinking, consequently, is haphazard or random rather thanselective. Changes in strength of curing agents that take place duringstorage and variations in commercial products further complicate thematter of securing adequate and uniform crosslinking.

The production of sound thick-section products through mold-curing andpostcure heat-aging is virtually impossible to achive primarily becausethe chemical residues of heretofore customary curing agents whenempolyed in accordance with heretofore customary procedures can notescape during mold-curing, and, when they escape from the interior of athick-section mold-cured elastomer during postcure heat-aging, theycause blowing and delamination and even depolymerization of the base gumthroughout the center portions of thick-section moldings.

We have found it to be advisable to control the postcure heat-agingresults through the control of the unsaturated hydrocarbon groups. Anexcess amount of an unsaturated hydrocarbon group (such, for example, asa vinyl group) relatively to the amount of curing agent employed may, inthe postcure heat-aging treatment, result in the production of postcureheat-aged products having undesirable properties. Therefore, inpracticing our invention, we prefer to employ amounts of curing agentsat least sufficient to effect utilization for crosslinking of all of theunsaturated hydrocarbon groups present in the gum stock employed. Inother words, we prefer to emloy gum stocks (as blends or copolymers)that contain unsaturated hydrocarbon groups in amounts sufiicient toeffect the degrees of crosslinking desired and to employ a curing agentin the heat-curing treatment in each case in amount sufficienteffectively to cause all, or substantially all, of the unsaturatedhydrocarbon groups to function as crosslinking agents or agencies inorder that no appreciable amount of unsaturated hydrocarbon pendantgroup or groups will be present in the heat-cured product subjected topostcure heat-aging.

The use of hydrocarbon-substituted polysiloxanes containing unsaturatedpendant hydrocarbon groups in accordance with our invention permits theutilization of procedures and processes devoid of the deficiencies ofthe heretofore customary processes and procedures. Polysiloxanes of theaforementioned types, containing unsaturated pendant hydrocarbon groups,may be employed with the greater advantage when employed in conjunctionwith a dialkyl peroxide curing agent in accordance with the invention.

Dialkyl peroxides employed by us react selectively to give crosslinkingat the positions of the unsaturated hydrocarbon pendant groups. Duringthe course of our research and development operations, the greateramount of our work was carried out with polysiloxanes consisting largelyof dimethyl siloxane units and containing ethyl vinyl siloxane units andwith the use of the dialkyl peroxide, di-tertiary-butyl peroxide ascuring agent. Consequently, our recorded data showing the results of theuse of such polysiloxanes and such dialkyl peroxide are the morecomplete, and, therefore, we shall include these data in tables at theend of this specification, and we shall refer to those tables in ourdiscussion of various points hereinafter.

The selectivity of dialkyl peroxides as curing agents,

18 referred to above, is demonstrated by the data set forth in TablesIII, IV and VIII.

The tensile strength and hardness data set forth in Table II show lowdegrees of cures of linear polysiloxanes consisting essentially ofdimethyl siloxane units, as compared with degrees of cures of linearpolysiloxanes consisting essentially of large numbers of dimethylsiloxane units and small numbers of ethyl vinyl siloxane units. Theproducts obtained in curing the linear polysiloxanes consistingessentially of dimethyl siloxane units were of very low quality, whereasthe products obtained in ouring the linear polysiloxanes consistingessentially of dimethyl polysiloxane units and ethyl vinyl siloxaneunits were of high quality.

Further evidence of the selectivity of the dialkyl peroxide,di-tertiary-butyl peroxide, is presented by the elongation and hardnessdata set forth in Table IV, which show that, for a linear polysiloxanegum containing a particular number of pendant vinyl groups,substantially the same degree of cure is accomplished regardless of theamount of curing agent employed within practical limits. The data setforth in Table IV demonstrate, also, the nonselective action of thehighly reactive organic peroxide, benzoyl peroxide. The data showingincreasing hardness and decreasing elongations of the products obtainedin using increasing amounts of benzoyl peroxide indicate thatcrosslinking was effected between methyl groups as well as through vinylgroups.

The data set forth in Table VIII further demonstrate the selectivity ofdi-tertiary-butyl peroxide. The increasing hardness of the curedelastomers resulting from increases in the numbers or concentrations ofvinyl groups show that close control of the state of cure of anelastorner can be accomplished through control of vinyl groupconcentrations in linear polysiloxane gums to be cured.

The data set forth in Tables V and X show that the compositions of asaturated hydrocarbon group may have little effect on some properties ofa cured elastomer.

The data set forth in Table VI show the effectiveness of the unsaturatedcyclohexenyl group in crossllnking.

The data set forth in Table VII shows the effectiveness of unsaturatedgroups in the curing of copolymers containing aryl groups and saturatedgroups other than the methyl group.

As demonstrated by the data set forth in Table IX, crosslinking probablyis accomplished through a mechanism involving the conversion of curingagents to free radicals which function selectively to remove hydrogenatoms from pendant methyl groups and other saturated groups, whichmodified saturated groups become adducts to pendant vinyl or otherethylenically unsaturated pendant groups, forming hydrocarbon linkagesbetween the linear polysiloxane chains. This conclusion is supported bythe fact that a blend of (1) a completely hydrocarbonsubstituted linearpolysiloxane whose pendant groups consisted entirely of methyl groupsand (2) a completely hydrocarbon-substituted linear polysiloxanecopolymer containing both methyl groups (in large amount) and vinylgroups (in small amount) was compounded with and cured byditertiary-butyl peroxide with the production of elastorners havingphysical properties equivalent to those obtained in curing a compound inwhich the only polysilox ane present was completelyhydrocarbon-substituted. linear polysiloxane copolymer that consistedessentially of (I) dimethyl siloxane units and (2) vinyl groups inamount equivalent to the amount present in the blend of linearpolysiloxanes.

The conclusion is supported, also, by the fact, as demonstrated by thedata of Table IX, that a blend of 1) a linear completelyhydrocarbon-substituted polysiloxane whose pendant hydrocarbon groupsconsisted only of methyl groups and (2) a cyclic polysiloxane whosependant groups included two vinyl groups per molecule was cured bydi-tertiary-butyl peroxide with the production of elastomers havingsatisfactory physical The significant beneficial effects of employingprecure aging treatments of the invention are shown by the data of TableXI. These data indicate that, generally, cured elastomers having lowcompression sets are not produced when an organic peroxide of any kindis employed for curing compounds in which the gum stocks employedconsist of completely hydrocarbon-substituted polysiloxanes whosependant groups consist entirely of saturated hydrocarbon groups. Thesedata show, also, that gun stocks comprising unsaturated hydrocarbonpendant groups cured with a dialkyl peroxide produce silicone elastomershaving the better compression set properties, and, also, that gum stockscomprising unsaturated hydrocarbon pendant groups produce siliconeelastomers cured with any of the dialkyl peroxide curing agents havingcompression set properties better than those of elastomers produced incuring with other organic peroxides gum stock consisting essentially ofcompletely hydrocarbonsubstituted polysiloxanes whose substituentsconsist essentially of saturated hydrocarbon pendant groups such, forexample, as methyl groups. Furthermore, the data indicate that thecompression set properties become less desirable substantially ininverse proportion to the amounts of other organic peroxides employedfor curing.

The data in Table XII show that the compression sets of polysiloxanegums of the invention that contain unsaturated pendant groups decreasein substantially inverse proportion to the amounts of unsaturatedhydrocarbon groups contained in gum stock used in producing compounds tobe cured.

Table XIII presents data illustrating the inability of polysiloxane gumscured in accordance with conventional heretofore customary procedureseffectively to produce elastomers when compounded with carbon black andconventional curing agents, or even with a dialkyl peroxide curingagent.

The data set forth in Table XIV demonstrate the effectiveness of dialkylperoxides of promoting and effecting the production of high-qualitycarbon black-filled silicone elastomers when employed in compounds to becured in conjunction with carbon black and with gum stocks consistingessentially of complete hydrocarbonsubstituted linear polysiloxanesconsisting essentially of (1) saturated hydrocarbon substituent (orpendant) groups (in large proportion) and (2) unsaturated hydrocarbongroups (in small proportion).

Table XV sets forth data showing results obtained in employing a numberof carbon black products that are commercially available.

Table XVI shows the characteristics of available carbon black productsand the qualities, cures and reinforcement obtained in efforts made toemploy them in the production of silcone elastomers through compoundingwith a linear polysiloxane gum and a dialkyl peroxide, subsequently,subjecting the compounds to heat-curing and heat-aging treatments.

Table XVII sets forth data illustrating the advantages to be derivedfrom the use of precure heat treatments and the use of alkalineneutralizing agents (or acid acceptors) in so conditioning carbon blackproducts as to eliminate inherent disadvantageous properties and makethem amenable for use in the production of carbon blackfilledelastomers.

The data set forth in Tables XVIII and XIX show the striking advantagein thick-section curing that may be obtained in employing the linearpolysiloxane gums of the invention and dialkyl peroxide curing agents,as compared with the virtually completely negative results obtained inutilizing gums, compounds and procedures in accordance with heretoforecustomary practices.

Table XX sets forth data showing the superiority of silicone elastomersof the invention over heretofore customary elastomers in resistance todeterioration under the influence of hydraulic brake fluids.

Table XXI sets forth data showing the capacities of silicone elastomersof the invention to resist deterioration when subjected to the action ofsaturated steam (water vapor) at an elevated temperature.

Table XXII has been presented for the purpose of illustrating thesubstantial equivalency among dialkyl peroxides employed as curingagents in practicing our invention.

GLOSSARY In the foregoing examples and the following tables, thefollowing terms and expressions, where employed, are to be interpretedas indicated below.

(A) Miniature penetrometer.-The miniature pene trometer used indetermining the hardness of silicone gums is a modification of thestandard miniature penetrometer used in measuring the hardness orviscosity of a plastic substance, such as asphalt, made in accordancewith suggestions contained in the article Miniature Penetrometer forDetermining the Consistency of Lubricating Greases by Kaufman, Gus;Finn, W. J. and Harrington, R. 1., Industrial and Engineering Chemistry,Analytic Edition, 11, 108-110 (1939).

In the modified miniature penetrometer, an aluminum plunger andpenetrometer cone weighing 20 grams has been substituted for the steelplunger and penetrometer cone, weighing 150 grams, of the standardminiature penetrometer. Otherwise, the modified miniature penetrometeris of the same structure and dimensions as that described in theaforementioned article.

Silicone gum stock is tested for hardness by lowering the penetrometercone with the plunger into contact with the surface of the gum stockwith the indicator reading zero. Then the penetrometer cone with itsplunger is released to permit downward movement under the influence ofgravity for a period of 10 seconds, and the depth of penetration isshown in millimeters on an indicator associated with the device. Theindicated penetration is identified as the miniature penetrometerreading (MPR).

(B) Compression set (ASTMD39552T).Degree of failure of a sample toreturn to its original size after removal of a deforming force.

Compression set tests are run by compressing a 1.129 inch diameter x0.500 inch high cylindrical specimen either under a constant load,method A, or at a definite fixed deflection, method B. After thespecimen has been compressed, it may be subjected to an elevatedtemperature for a fixed time (usually 22 hrs. at 70 C.), then the loadis released; after a 30 minute rest, the permanent. change in the heightof the specimen is measured and the percent set calculated. A smallvalue is desirable.

Compression set is expressed as percent of original deflection in methodB.

Compression set is expressed as percent of original thickness in methodA.

(C) Elongation (ASTM D41251T).Amount of stretch of a sample under atensile force expressed as a percentage of the original length.

(D) Hardness (ASTMD67649T).-Degree of indentation produced by a plungeror indentor under a specific load. Measured with a Shore A Durometer.The values range from O to maximum hardness of 100.

(E) Set at break.Measure of the permanent set of a specimen after beingstretched to the breaking point, obtained by piecing the broken partstogether and measuring the distance between the bench marks. Change isexpressed in percent of the original length.

(F) Tear strength.-Similar to tensile test, except that a differentright angle dumbbell shape is used. Sample tears at the right angle.Force required to tear specimen divided by thickness is tear strength(lb./in.).

(G) Tensile strength (ASTMD41249T).The force necessary to rupture arubber specimen when stretched to the breaking point divided by theoriginal cross sectional area (lb./in.

(H) Williams plasticity numben-The thickness of a cylindrical sample ofuncured elastomer after being deformed under a known weight. Therecovery value represents the amount of return of the rubber after theweight has been removed.

In processes of the invention involving postcure heataging, heat agingmay be carried out in any manner suitable for accomplishing the desiredresult, and any suitable maximum temperatures and any suitable periodsof time may be employed. Thus, for example and by way of illustration,when a maximum temperature of about 480 F. is specified, the curedelastorner may be placed directly in an environment heated to andmaintained at a temperature of 480 F. and permitted to reside in thatenvironment until it reaches a temperature in equilibrium with thetemperature of the environment, or, the temperature of the environmentmay be increased one or more times or decreased one or more times duringa period of either continuous or intermittent residence of the elastomertherein. On the other hand, the elastomer may be placed, initially in anenvironment having a temperature lower than 480 F., and, thereafter, thetemperature of the environment may be raised step-wise, or from time totime, gradually or rapidly to a temperature of 480 F. The specifiedtemperature of 480 F. is critical only to the extent that it representsthe usual maximum temperature to which silicone elastomers may besubjected when employed in industry. When lower temperatures ofemployment are contemplated, lower postcure heat-aging temperatures maybe employed, and, when higher temperatures of employment arecontemplated, higher postcure heat-aging temperatures may be employed.The postcure heat-aging temperature and time employed with respect toany particular elastomer, if any, will be determined largely on thebasis of the natures of volatile materials to be eliminated and thecontemplated temperature of employment and, of course, on the basis ofeconomic considerations.

It is to be understood that the products of the invention include, amongothers, siloxane gums of particular compositions containing, or free of,curing agent; compounds containing siloxane gum and filler material andcontaining, or free of, curing agent; precure-aged compounds, aged inany manner for any suitable periods of time as, for example, roomtemperature-aged compounds, aged for periods of time ranging from afraction of an hour to a week or more and heat-aged compounds, aged atany suitable temperature for periods of time ranging from a fraction ofan hour to several hours or several days; elastomers of all types,including carbon black-filled elastomers, elastomers filled withnon-carbonaceous fillers and elastomers filled with both carbon blackfillers and non-carbonaceous fillers; thick-section cured elastomers;thick-section cured and heat-aged elastomers; elastomers havingparticular physical properties; elastomers of all types cured withparticular curing agents; and compounds, cured and uncured, formed withparticular types of gums.

Because the phenyl group is relatively stable under conditions employedin carrying out a process of our invention, we consider that group to bea saturated group for the purposes of our invention, and we haveincluded it among the saturated hydrocarbon groups through thespecification and claims.

The data set forth below illustrate the selectivity of dialkyl peroxidesin effecting crosslinking through unsaturated hydrocarbon groups.

Heat-curable compositions were formed by dispersing a silica filler anda dialkyl peroxide curing agent in (1) a siloxane gum consistingessentially of dimethyl units (identified below, under siloxane gum, asdimethyl) and in (2) a siloxane copolymer gum consisting essentially ofdimethyl siloxane units (99.85 mole percent) and ethyl vinyl siloxaneunits (0.15 mole percent) (identified below, under siloxane gum, ascopolymer) in a Banbury mixer.

The silica filler was employed in amount equal to 43 parts by weight foreach parts by weight of siloxane gum.

The dialkyl peroxide, di-tertiary-bntyl peroxide curing agent wasemployed in the amounts by weight indicated below, under curing agentconcentration per 100 parts by weight of siloxane gum.

TABLE III Elastomor properties (After 25 4 minutes at 340 F. Mold-cureCuring followed by 24 hours. Oven Siloxane gum agent Post-cure at 480F.)

concentration Tensile Elongation Hardness (p.s.1.) (percent) (Shore A)Dimethyl 0. 6 154 66 Oopolymer 0. 4 895 250 78 Et-Vt C0polymer 0.6 828250 77 2 WEEKS ROOLI TEMP. AGING OF UNOURED STOCK.

(CURING AGENT ADDED AFTER AGING) Dimethyl 0. 6 255 380 31 Do 1.0 205 34033 PRECURE HEAT AGING 2 HRS. AT 300 F. OF GUlM-i-FILLER The followingdata illustrate the effects of different curing agent concentrations atconstant concentrations of unsaturated hydrocarbon substituent groupswith respect to each of two siloxane gums containing differentconcentrations of unsaturated hydrocarbon substituents.

A finely divided, solid silica filler and two different curing agentswere dispersed in the two different siloxane gums in a Banbury mixer toproduce heat-curable compositions. The silica filler was dispersed ineach of the siloxane gums in an amount by weight equal to 43 parts per100 parts by weight of gum. The curing agents employed were benzoylperoxide and the dialkyl peroxide, di-tertiary-butyl peroxide. Theamounts by weight of curing agents employed per 100 parts by weight ofsiloxane gum and the types of curing agents employed are indicated belowunder the headings, curing agent concentration, and curing agent,respectively.

The compositions produced by mixing the components were precureheat-aged for 2 hours at 300 F., the heataged compositions weresubjected to mold cure treatments, and the mold cured compositions weresubjected to postcure heat treatments for 24 hours at 480 F.

TABLE IV Elastomer properties (after mold- Curing cure followed by 24hour oven agent postcure at 480 F.) Curing agent concentration TensileElongation Hardness (p.s.i.) (percent) (Shore A) Siloxane gum consistingessentially of dimethyl siloxane units (99.85 mole percent) and ethylvinyl units (0.15 mole percent):

Di-tertiary-butyl peroxide 0. 750 220 42 Do 0. 40 740 230 44 Do 0.60 720220 46' Benzoyl peroxide 0. 20 490 270 30 D0 0. 40 750 230 38 D0 0.60820 180 46 Siloxane gum consisting essentially of dirnethyl siloxaneunits (99.44 mole percent) and ethyl vinyl siloxane units (0.56 molepercent):

Di-tertiary-butyl peroxide 0.20 770 100 60 Do 0.40 725 80 62 Do 0.60 74080 62 Benzoyl peroxide 0.20 540 220 37 D 0.40 750 170 48 Do 0.60 720 12055 TABLE V TABLE VI Comparison of various vinyl organic siloxaneadditives 25 for dimet'hyl siloxane gums [Banbury mixed: Precureheat-aged 2 hours at 300 F.] Composition (parts by Weight) 100 parts gum45 parts Santocel CS (silica filler) 0.7 part DTBP (di-tertiarybutylperoxide) (Cyclohexenyl RSiO) as the crosslinking group [Roll millmixed: Immediate cure] Composition (parts by Weight) 100 parts gum partsSantocel CS (silica filler) Concentra- Elastomer properties (mold-cure0.7 part L'IBP (di-tertiarybutyl peroxide) tion of vinyl followed by 24hour oven postcure Vinyl or anic additive in at 480 F.) siloxane aditive gum (mole percent) T '1 El t' H d 9115} 6 0118a 1011 11955Elastomer properties after mold-cure (p.s.i.) (percent) hore A) 35followed by 24 1 0 1 1- postcure at 480 Mole percent of cyclohexenylethyl siloxane in gum C H Tensile Elongation Hardness si o 0. 15 765 27045 40 (p.s.i.) (percent) (Shore A) I CH=CH2 0.15 544 475 68 0.50 705 12585 (3H3 0.0 (Dimethyl Gum) No cure Si0 0. 15 925 250 C H CH; 45 O Thefollowing table, Table VII, includes data obtained in employingcopolymers compn'sing sil o 780 250 Paradimethylsilylenphenyldimethylsiloxane units,

CH= CH: which units can be identified as None (Dimethyl Bis-para(dimethyl oxysilyl) benzene units and they Gum) 0. 0 100 140 28 havebeen so identlfied 1n the appended claims for convenience.

Composition (parts by weight):

100 parts gum TABLE VII Vinyl cures 09 various copolymers[Ro1lmill:mixed: Immediate cure] Elastonrer properties (after mold-Ethyl vinyl siloxane cure+24 hr. postcure at 480F.)

content (mole Polymer composition (mole percent) percent) TensileElongation Hardness (p.s.i.) (percent) (Shore A) 0.18 3% diphenylsiloxane units, 96.82% dimethyl 885 250 siloxane units. 0.18 5% diphenylsiloxane units, 94.82% dimethyl 935 250 70 siloxane units. 0.18 1%phenyl methyl siloxane units, 98.85% di- 868 200 methyl siloxane units.0.18 4% phenyl methyl siloxane units, 95.85% di- 005 285 77 methylsiloxane units. 0.15 1% para-dimethyl-silylenphenyldimethylsilox- 745260 75 ane units, 98.85% dimethyl siloxane units. 0.15 4%para-dimethyl-silylenphenyldimethylsilox- 748 285 75 ane units, 95.85%dimethyl siloxane units. None 1 diethyl siloxane units 2 1073 2 410 l 251 Precure heataged 1 hour at 300 F. 2 Mold cure properties only with 1.0part DTBP.

The para-dirnethylsilylenphenyldirnethylsiloxane units referred to abovemay be represented by the structural formula:

TABLE XI Efiect of curing agents and polymers on compression set CH; CH:[Dimethyl polysiloxanc gum (identified below as Dimethyl Gum) and Idirnethyl polysiloxane gum containing ethyl vinyl siloxane units g CH(identified below by the mole percent concentration of ethyl vinyl 3 3siloxane units)] Benbury mixed: 1 100 parts gum 43 parts Sentocel CS(silica filler) TABLE VIII E [feet of varying vinyl group concentrationsCuring agent Compression {l 3anbury mixed: Precure heat-aged 2 hours at300 F.] Ethyl vinyl Siloxane 388 2 3: 53% by Welght): concentrationConcentration 96 hr. at 350 F.)

43 p b i. n (mole percent) Kind (parts per 100 (except as indipzirtsSantocel CS (silica filler) arts um both cat d) t 0.4 port DTBP(di-tertiary-butyl peroxide) p by iveight) of g gz i gg fiection ConcenlConcen- Elastomer properties (after mold-cure (tiratiorkil all itgatlion all followed by 24 iigigrlgven postourc at A Immediate cureimet y et 1y viny e0 snomne Silomue Dunethyl Gum DTBP 1. 0 Only slightunits units DTBP 2 O g- (mole (mole Tensile Elongation Hardness TB PB 0percent) percent) (p.s.i.) (percent) (Shore A) 0 69 (29 at I 350 If).99. 90 0. 10 g g ago 0 g 99.85 0.15 s 2 0 99. 44 0. as 723 so 62 32202-0 I- n1; DTBP 0.6 as. 0. 1.0 31. 3O 0. 6 42. TABLE 1X Q: 5:2 22: 0.15 1.O 68. Miscellaneous cures through the vinyl group agga ffggg .DimcthylGum DTBP 2.0 Only slight [Roll mill mixed: Precure heat-aged 2-6 hoursat 300 Fe] cure. Composition (parts by weight): TBPB 2. 0 7332203 211;

100 parts gum 35 parts Sentocel (38" (silica filler) B2202... 2.0 85 (22hr. at 0.7 port DTBP (di-tertiary-butyl peroxide) 0 DTBP o 6 23350 F.).

0: DTBPIIII 110 22: Q Elastorncr properties (After mold- P 0. 6 28. curefollowed by 24 hour oven 40 0. TBPB 1.0 35. Composition of SpecialPostcure at at 480 F.) BZ2O2 O. 6 33. hose additive to 0. BZ202 1.0 47.polymer compound 0. Precure He rst-aged Tensile Elongation Hardness at300 J (p.5.1.) (percent) (Shore A) g y Gum--- ggggnu 5:8 210 92:Dimethyl None so 190 as 4D 0. a 82. D0 0.70 Part Of 500 500 39 2. 0 89.tetramer 1. 0 19. (EtViSiO); 1. 0 46. (Me SiOh. 1.0 57. Blend 1 None 750250 51 2.0 83.

i 1 The blend consisted of 66 parts by weight of dimethyl siloxane gumD0 y peroXideand 34 parts by weight of copolymer gum containing 0.56mole percent of TBPB y D F ethyl vinyl silornne units and 99.44 molepercent of dimethyl siloxaue z 2= y p r xide. units. Perts=parts byweight.

TABLE X 55 Compression set of vinyl-DTBP cured elastomers [Roll millmixed: Precure heat-aged 2 hours at 300 F.] TABLE XII Composition (partsby weight):

100 parts gum 45 parts Santocel "cs" Sines filler) 0.7 port DTBP(di-tertinry butyl peroxide) Compression set (After 24 hours Postcure at480 F.)

Eflect of vinyl concentration on compression set Polymer composition(mole percent) (Method B, ASTM 06 part DTBP (di-tertiary-butyl peroxide)395-521 hours at 300 F. percent of original deflection) 0.15% ethylvinyl siloxzme units 99.85% dimethyl 12 Ethyl vinyl siloxane conoentra-Compression set (Method B, ASTM siloxene unites". 70 tion (mole percent)395-521 70 hours at 350 F.) (Percent 0.15% methyl vi s, 9 of originaldeflection) dirnethyl slloxene units 9 0.15% plienyl vinyl siloxaneunits, 99.85%

dimethyl siloxsme units l0 0. 10 30 dimethyl siloxane units (cured with2.0 0. 15 18 parts benzoyl peroxide) 72 O. 56 11 TABLE X111 PARTI-CARBON BLACK-FILLED COMPOUNDS WITH CONVENTIONAL CURES Polymer Moldcure Curing agent Filler (parts per 100 composition (minutes (parts per100 parts gum, both by (mole percent) at F.) parts gum, weight) both byweight) A Dimethyl at 250 5.0 B2202 25.0 Furnex (SRF). B.-. do at 3 3.0DTBP Philblaek A (MAF).

3.0 D Do. 3.0 D Do. .3 B Do. .0 B 40 Philblaek O (HAF). .0 B Do. .0 BD0.

PART IIELASTOMER PROPERTIES Mold cured Posteured Tensile ElongationHardness Tensile Elongation Hardness (p.s.i.) (percent) (Shore A)(p.s.i.) (percent) (Shore A) 2 Copolymer consisting essent1 siloxaneunits (0.19%).

3 No cure.

l Undercured; high set at break76%.

TABLE XIV ally of dimethyl siloxane units (99.81%) and ethyl vinylCarbon black-filled compounds with DTBP-vinyl cures PART IA" MOLD-GURED15 MINUTES AT 250 F., OTHERS MOLD-CURED 25 MINUTES AT 340 F.

Polymer composition (mole percent) Catalyst (parts per 100 parts byweight) Filler (parts per 100 parts by weight) 40 Philblack A (MAF) 40Vulcan 3 (HAF).

40 Sterling V (GPF). 40 Continex (HAF). 40 Aromex (HAF).

PART II-ELASTOMER PROPERTIES Mold cured Postcured Tensile ElongationHardness Tensile Elongation Hardness (p.s.i.) (percent) (Shore A)(p.s.i.) (percent) (Shore A) 1 No pure.

TABLE XV Properties of various types of carbon black filled elastomers[Roll mill mixed: 24 hours room temperature aged] Composition (parts byweight):

100 parts 0.19 mole percent ethyl vinyl siloxane units 40 parts carbonblack 2.0 parts DTBP (di-tertiary-butyl peroxide) Part IMold-cureproperties Part IIPstcured properties min. at 340 F.) (24 hoursoven-cure at 480 F.) Carbon black Type Te nsile Elongation HardnessTensile Elongation Hardness (p.s.i.) (percent) (Shore A) (p.s.i.)(percent) (Shore A) Mieronex Mark II 1 No cure.

TABLE XVI Relatwnshzp of carbon black propertles to curingcharacterlstzcs mi Et-Vz gum wzth DTBP Particle Volatiles Name Type pHSize (A.) (wt. per- Cure cent) Mieronex Mark II No Cure Micronex 4, 4280 5. 5 Do. Micronex W-6 4. 8 290 5. 5 D0. Spheron N 4. 5 220 5.0 Do.3. 8 345 5. 0 D0. 4. 5 300 6. 0 D0. 9. 3 270 1. 2 D0.

Cure (medium reinforcement). 9. 3 430 0 5 D0. 9. 5 580 0. 5 D0. 10. O850 0. 5 Do.

8. O 1, 500 O 3 Core (low rein- Iorcement). 7. 6 3, 000 D0. 9. O 280 1.0 D0. 9.1 360 1. 1 Cure (good reinforcement) Philblaek A 9. 7 590 l. 1D0, Gontinex. 9. 5 330 2. 0 D0. Sterling V t 745 D0. Aromeru 9 3 300 l 2D0. Vulean3 9 0 365 1 0 Do.

TABLE XVII percent of ethyl vinyl siloxane units and 99.81 mole Use ofprecure heat-aging and aczd acceptors in carbon percent (approximately)dunethyl 8110mm umts' black filled compounds 40 parts by weight ofMlCDOIlX W6. 2 parts by Weight of di-tertiary-butyl per-oxide. RECIPESAND HEAT TREATMENTS (Subjected to precure heat-aging treatment for 2hours (A) 100 parts by Weight of gum containing 0.19 mole at R prior toheat curing treatment) percent of ethyl vinyl siloxane units and 99.81mole percent (approximately) of dimethyl siloxane units. 40 (C) Parts yWeight 9 8 a l g 0. 9 m le parts b i ht f Mi o W 6 2 parts b weightpercent of ethyl vinyl s1loXane units and 99.81 mole ofdi-tertiary-butyl peroxide. percent (approximately) of dimethyl siloxaneunits.

40 parts by Weight of Micronex W-6. 2 parts by (Sub ecteG immediately toheat-curing treatments.) Weightof (1H6 marybutyl Peron. d6 5 parts yweight (B) 100 parts by weight of gum containing 0.19 mole 7 of WitcarbR (finely divided calcium carbonate).

(Subjected immediately to heat-curing treatment.)

Mold-cured Properties (30 minutes Posteure Heat-aged Properties (24 at340 F.) hours at 480 F.)

Tensile Elongation Hardness Tensile Elongation Hardnesss (p.s.1.)(percent) (Shore A) (p.s.i.) (percent) (Shore A) A 1 B 390 275 as 490220 55 C 420 470 28 370 165 66 1 No cure.

TABLE XVIII Heat-aging of dimethyl gum thick section moldings [Bamburymixed Precure heat-aged 2 hours at 200 F. (100 parts by weight of gumconsistingessentially of dimethyl slloxane units. 43 parts Santocel CS)Mold-Cure Properties Curing Agent ffi A) Delamina,

Center Distion and coloration Blowing Side Center 0.6 Parts Benzoyl 4040 None None.

Peroxide. 2.0 Parts Tertiary- 42 42 do D0.

butyl Perbenzoate. 2.0 Parts Di-tertiary- 41 41 do Do.

butyl Peroxide.

Postcure Heat-Aged Properties (24 hours at 480 F.)

Curing Agent Hardness (Shore A) At Center Dis- Dela-minacoloration tionand Blowing Side Center 0.6 Parts Benzoyl 55-60 Very Much Much.

Peroxide. so 2.0 Parts Tertiary- 55-60 do do Do.

butyl-Perbenzoate. 2.0 Parts Di-tertiary- 45 do do Do.

butyl Peroxide.

TABLE XIX Heat-aging of vinyl-c0ntaining gum thick-section moldings [100parts by weight of gum containing 0.15 mole percent ethyl vinyl siloxaneunits and 99.85 percent (approxmately) of d methyl siloxane units. 43parts by weight Santocel CS (finely divided silica)] POS'ICURE HEAT-AGED24 HOURS AT 480 F.

Post-cure Heat-aged Properties Hardness (Shore) At Delam- CenterDisination coloration and Side Center Blowing 55 Very soft Much Much.

57 d do Do.

55 35-.. Slight None.

55 44- Very slight--. Do.

70 do Do.

TABLE XX The data contained in this table demonstrate the superiorquality of resistance to hydraulic brake fluids of silicone elastomersof the invention, as compared with heretofore customary types ofsilicone elastomers.

Three elastomers were prepared and tested. In the preparation of one,gum stock A, which consisted essentially of linear dimethylpolysiloxanes was employed. In the preparation of another, gum stock B,a copolymer of the invention, comprising linear polysiloxanes consistingessentially of dimethyl siloxane units (99.65 percent by weight) andethyl vinyl siloxane units (0.35 percent by weight=0.26 mole percent)was employed. In the preparation of the third, gum stock C, a copolymerof the invention, comprising linear polysiloxanes consisting essentiallyof dimethyl siloxane units (99.80 percent by weight) and ethyl vinylsiloxane units (0.20 percent by weight) was employed.

The elastomers were prepared and tested in accordance wtih theprocedures and with the results indicated by the data set forth below.(The parts referred to are parts by weight.)

HA! B 071 Composition of Elastomer:

Gumstock, parts 100 100 Santocel CS, parts 45 40 45 Superfloss, parts 105 10 Superpax, parts 3 3 Benzoyl peroxide, par 2 Di-tertiary-butylperoxide, part 0.8 0.8 Cure, Time in minutes 15 25 25 Temperature, F-250 340 340 Postcure Heat-aging propertie Hardness (Shore A) 76 72 76Tensile (p.s.i.) 480 940 870 Elongation (percent) 230 230 ImmersionTests:

Brake Fluid Delco 9 Delco 9 Delco 9 Fluid Temperature, C 96 96 96 DaysImmersed 10 10 10 Final Properties:

Hardness (Shore A) 62 68 70 Tensile (p.s.i.) 140 795 820 Elongation(percen 60 230 240 Extent of Change:

Hardness (Shore A Units) 14 4 6 Tensile (p.s.i.) 71 15 6 Elongation(percen 57 0 0 Weight Gain (percen 2. 5 4. 5 4.3 Weight loss of rubberat r heating in air and circulating oven for one hour at 300 F. C.) 1.80 0 4. 3 4. 3 4. 0

33 TABLE m The data set forth below illustrate capacity of siliconeelastomers of the invention to resist deterioration under the influenceof saturated steam (water vapor). Two different polysiloxane gum stockswere employed in preparing elastomers for testing purposes. One gumstock, A, comprised linear polysiloxanes consisting essentially ofdimethyl siloxane units. The other gum stock, B, comprised linearpolysiloxane copolymers of the invention consisting essentially ofdimethylsiloxane units (99.65 percent by weight) and ethyl vinyl units(0.35 percent by weight).

One recipe containing gum stock A, was employed, benzoyl peroxide beingincorporated and two recipes employing gum stock, B, were employed, onehaving benzoyl peroxide incorporated therein and the other havingditertiary-butyl peroxide incorporated therein. Otherwise,

34 TABLE XXII This table is presented for the purpose of illustratingthe substantial equivalency among dialkyl peroxides employed inpracticing our invention.

The data set forth below were obtained through tests conducted onelastomers produced in compounding, immediate mold curing after millingand postcure heataging of recipes containing the indicated components inthe proportions, for the periods of time and at the temperatures setforth.

In each instance, there was employed a recipe containing a polysiloxanegum comprising linear polysiloxanes consisting essentially of dimethylsiloxane units (99.65 percent by weight) and ethyl vinyl siloxane units(0.35 percent by Weight=0.26 mole percent), Santocel CS filler and adi-tertiary-butyl peroxide curing agent of the type the recipes wereidentical. indicated. (All parts referred to are parts by weight.)

SECTION I Amount of Amount of Type 01 Peroxide Peroxide in Filler Em-Time and Tempera- Employed parts per 100 ployed in ture Employed inparts of Gum parts per 100 Mold Cure parts of Guru A lTertiary-butyl-tertiary 1. 29 85 minutes at 340 F.

triptyl peroxide. B Diterti ary-butyl 1. 00 25 minutes at 340 F.

peroxide. C- Tertiary-butyl-tertiary- 1.03 25 minutes at 340 F.

triptyl peroxide. D Di-tertiary-butyl 0.80 40 25 minutes at 340 F.

peroxide. E Tertiary-butyl triethyl 1. 29 4O 25 minutes at 340 F.

methyl peroxide. F Tertiary-butyl triethyl- 1. 29 40 [30 minutes at 340I methyl peroxide. G Tertiary-butyl trietllyl- 1. 29 4O 25 minutes at356 F.

methyl peroxide.

SECTION II AFTER MOLD-CURE After Post-cure Heataiging at 480 F. for 24hours Tensile Elongation Hardness Tensile Elongation Hardness Strength(percent) (Shore A) Strength (percent) (Shore A) (p.s.i.) {p.si.)

The procedures followed in producing elastomers and It is to beunderstood that the theories set forth herein in testing, and theresults obtained, are indicated by the data set forth below:

Recipe (parts by weight):

urnstoek Gumstocli B 100 Santocel CS 40 40 40 Di-tertiarydlut-ylperoxide 0. 8 Benzoyl peroxide. 2 0. 6 Postcure Heat-aged Pro Tensile(p.s.i.) 650 860 510 Elongation (perce 210 230 220 Hardness (Shore A) 5457 64 Weight (gr-(11115)- 4. 35 4. 28 5. 03 Thickness (inches) .076 .015O88 Properties After Test:

Pressure (p.s.i.g.) -c 100 100 100 Tlrne (Hoursh 22 23 24 Tensile(p.s.i.) -c 540 360 Elongation (percent) h 150 210 Hardness (Shore A) 2848 58 Weight (percent) 5. 66 4. 5. 45 Thickness (inches) .082 .011 089Change in Properties: I

Tensile (percent) 71 -31 ?g 6 Thickness (percent) +1 are provided forpurposes of illustration and explanation and not for purposes oflimitation.

What is claimed is:

1. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of (l) adiorganopolysiloxane gum selected from the group consisting ofpolysiloxane polymers and copolymers Whose combined organic substituentsare all bonded through silicon-carbon linkages and consist entirely ofsaturated hydrocarbon radicals; (2) a vinylcontaining organosiloxaneselected from the group consisting of polysiloxane polymers andcopolymers in which the vinyl radicals and any other organic radicalspresent therein are also bonded through silicon-carbon linkages; and (3)an organic peroxide curing agent.

2. A heat-curable gum stock composition for use in the production ofsilicone elastorners that comprises a mechanical mixture of (l) adiorganopolysiloxane gum selected from the group consisting ofpolysiloxane polymers and copolymers whose combined organic substituentsare all bonded through silicon-carbon linkages and consist entirely ofsaturated hydrocarbon radicals; (2) a vinyl-containing organosiloxaneselected from the group consisting of polysiloxane polymers andcopolymers in which the vinyl radicals and any other organic radicalspresent therein are also bonded through silicon-carbon linkages; (3) anorganic peroxide curing agent; and (4) a filler material.

3. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of (l) adiorganopolysiloxane gum selected from the group consisting ofpolysiloxane polymers and copolymers whose combined organic substituentsare all bonded through silicon-carbon linkages and consist entirely ofsaturated hydrocarbon radicals; (2) a vinyl-containing organosiloxaneselected from the group consisting of polysiloxane polymers andcopolymers in which the vinyl radicals and any other organic radicalspresent therein are also bonded through silicon-carbon linkages, whereinfrom 0.037 to 0.74 percent of the total silicon atoms in the mixture arebonded to said vinyl radicals; (3) a di-tertiary-butyl peroxide curingagent; and (4) a filler material.

4. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of (1) adiorganopolysiloxane gum selected from the group consisting ofpolysiloxane poly mers and copolymers whose combined organicsubstituents are all bonded through silicon-carbon linkages and consistentirely of saturated hydrocarbon radicals; (2) a polysiloxanecontaining ethyl-vinyl siloxane units in which the ethyl and vinylradicals are also bonded through sili con-carbon linkages; and (3) anorganic peroxide curing agent.

a 5. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of 1) adimethylpolysiloxane gum; (2) an ethyl-vinyl polysiloxane; and (3) anorganic peroxide curing agent.

6. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of (1) aphenylmethylpolysiloxane gum; (2) an ethyl-vinyl polysiloxane; and (3)an organic peroxide curing agent.

7. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of 1) adiphenylpolysiloxane-containing gum; (2) an ethyl-vinyl polysiloxane;and (3) an organic peroxide curing agent.

8. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of (1) adimethylpolysiloxane gum; (2) an ethyl-vinyl polysiloxane; and (13) adi-tertiarybutyl peroxide curing agent.

9. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of 1) aphenylmethylpolysiloxane gum; (2) an ethylvinyl polysiloxane; and (3) aditertiary-butyl peroxide curing agent.

10. A heat-curable gum stock composition for use in the production ofsilicone elastomers that comprises a mechanical mixture of (1) adiphenylpolysiloxane-containing gum; (2) an ethylvinyl polysiloxane, and(3) a di-tertiary-butyl peroxide curing agent.

11. A heat hardenable composition of matter comprising a mixture of (1)a diorganopolysiloxane gum in which the organic radicals are allattached to the silicon through silicon-carbon linkages, all of saidorganic radicals being free of aliphatic unsaturation, (2) a vinylcontaining organosiloxane in which all of the vinyl radicals and anyother organic radicals present in said siloxane (2) are attached to thesilicon through silicon-carbon linkages, the proportions of (1) and (2)being such that in the siloxane mixture from 0.037 to 0.74 percent ofthe total silicon atoms are bonded to vinyl radicals and (3) an organicperoxide vulcanizing agent.

12. A composition in accordance with claim 11 in which siloxane (1) is adimethylpolysiloxane gum.

13. A composition in accordance with claim 11 in which siloxane (1) is amethylphenylpolysiloxane gum.

14. A heat hardenable composition of matter comprising a mixture of (1)a diorganopolysiloxane gum in which the organic radicals are allattached to the silicon through silicon-carbon linkages, all of saidorganic radicals being free of aliphatic unsaturation, (2) a vinylcontaining organosiloxane in which all of the vinyl radicals and anyother organic radicals present in said siloxane (2) are attached to thesilicon through silicon-carbon linkages, the proportions of (1) and (2)being such that in the siloxane mixture from 0.037 to 0.74 percent ofthe total silicon atoms are bonded to vinyl radicals, (3) an organicperoxide vulcanizing agent and (4) a filler.

15. A composition in accordance with claim 14 in which siloxane (1) is adimethylpolysiloxane gum.

16. A composition in accordance with claim 14 in which siloxane (1) is aphenylmethylpolysiloxane gum.

17. A heat hardenable composition of matter comprising a mixture of (l)a diorganopolysiloxane gum in which the organic radicals are allattached to the silicon through silicon-carbon linkages, all of saidorganic radicals being free of aliphatic unsaturation, (2) a vinylcontaining organosiloxane in which all of the vinyl radicals and anyother organic radicals present in said siloxane (2) are attached to thesilicon through silicon-carbon linkages, the proportions of (1) and (2)being such that in the siloxane mixture there is from 1 vinyl radicalper 2700 silicon atoms to 1 vinyl radical per silicon atoms and (3) anorganic peroxide vulcanizing agent.

18. A composition in accordance with claim 17 in which siloxane (1) is adimethylpolysiloxane gum.

19. A composition in accordance with claim 17 in which siloxane (1) is amethylphenylpolysiloxane gum.

20. A heat hardenable composition of matter comprising a mixture of (1)a diorganopolysiloxane gum in which the organic radicals are allattached to the silicon through silicon-carbon linkages, all of saidorganic radicals being free of aliphatic unsaturation, (2) a vinylcontaining organosiloxane in which all of the vinyl radicals and anyother organic radicals present in said siloxane (2) are attached to thesilicon through silicon-carbon linkages, the proportions of (1) and (2)being such that in the siloxane mixture there is from 1 vinyl radicalper 2700 silicon atoms to 1 vinyl radical per 135 silicon atoms, (3) anorganic peroxide vulcanizing agent and (4) a filler.

21. A composition in accordance with claim 20 in which siloxane (l) is adimethylpolysiloxane gum.

22. A composition in accordance with claim 20 in which siloxane (1) is aphenylmethylpolysiloxane gum.

No references cited.

MURRAY TILLMAN, Primary Examiner.

1. A HEAT-CURABLE GUM STOCK COMPOSITION FOR USE IN THE PRODUCTION OFSILICONE ELASTOMERS THAT COMPRISES A MECHANICAL MIXTURE OF (1) ADIORGANOPOLYSILOXANE GUM SELECTED FROM THE GROUP CONSISTING OFPOLYSILOXANE POLYMERS AND COPOLYMERS WHOSE COMBINED ORGANIC SUBSTITUENTSARE ALL BONDED THROUGH SILICON-CARBON LINKAGES AND CONSIST ENTIRELY OFSATURATED HYDROCARBON RADICALS; (2) A VINYLCONTAINING ORGANOSILOXANESLECTED FROM THE GROUP CONSISTING OF POLYSILOXANE POLYMERS ANDCOPOLYMERS IN WHICH THE VINYL RADICALS AND ANY OTHER ORGANIC RADICALSPRESENT THEREIN ARE ALSO BONDED THROUGH SILICON-CARBON LINKAGES; AND (3)AN ORGANIC PEROXIDE CURING AGENT.